Аннотация:Calculations of interstellar-gas ionization by background radiation at the periphery of galaxies are performed which indicate a sharp cutoff in the neutral hydrogen surface-density distribution. A comparison with observational data establishes an upper limit of (0.5 to 5) by 10 to the -23rd power erg/sq cm per sec-Hz-sr for the intensity of the ionizing background radiation in the wavelength region between 100 and 912 A, which is not accessible to direct observation. It is noted that the uncertainty in the upper limit is associated with the lack of reliable information on the mass distribution and surface density of the interstellar gas in galaxies.